1. Field of the Invention
The present invention relates to a method of cooling hot-rolled rolling stock, wherein, for cooling preferably both sides of hot-rolled wide strip, a heat exchange is carried out between the rolling stock and the cooling medium by means of a pressurized water flow through a sequence of a pressure chamber and a convection chamber, and wherein the rolling stock is cooled by a targeted control of the intensity of the forced convection.
The invention also relates to an apparatus for carrying out the method.
2. Description of the Related Art
The findings in connection with cooling of hot-rolled rolling stock are based on practical experiences gathered over many years. These experiences show that, for example, in the case of wire and rod steel, significantly higher cooling intensities are achieved in water cooling sections than in water cooling sections of wide strip hot rolling trains. In cooling sections for wire and rod steel, the heat transmission coefficients are, for example, about 20,000 to 40,000 W/m.sup.2 K, while heat transmission coefficients of about 800 to 1,500 W/m.sub.2 K, i.e., a cooling intensity which is lower about ten to fifteen times, are achieved in a laminar cooling section following the finishing stand for hot-rolled wide strip. This has the result that a significantly longer cooling section is required for the same temperature drop in the case of hot-rolled wide strip as compared to wire and rod steel.
Because of more recent technologies in rolling hot-rolled strip, particularly for forming a ferritic structure, intensive cooling is required particularly between the last stands of a rolling train in order to achieve lower final rolling temperatures with stand spacings which are not too large. in the case of rolling thin dimensions, in which the hot-rolled strip reel must be arranged closer to the finishing stand for avoiding unsteady running of the strip, water cooling sections with higher cooling intensities are also required.
DE 39 27 276 A1 discloses a method of hardening steel with liquid cooling media. As mentioned in DE 39 27 276 A1, when using the previously known methods of hardening steel by means of liquid cooling media, only a small hardening depth in relation to the border zone is achieved in the rods having small rod diameters. In contrast, the method according to DE 39 27 276 A1 not only makes possibly a significant increase of the hardening depth, but also makes possible in a particularly simple manner a complete through hardening particularly of rod-shaped load products having diameters of up to 70 mm by subjecting the rolled product of steel immediately after the rolling process to high flow velocities in a cooling section provided with cooling media. The flow velocities in the cooling section are advantageously so high that heat transmission coefficients are greater or equal to 50,000 W/m.sup.2 K are produced and the rolling stock is cooled in the cooling section until the average temperature of the cross-section of the rolling stock is below the MS temperature, so that, after leaving the cooling section, the austenite which is still present in the core is converted into an intermediate structure (bainite) as a result of the temperature equalization across the cross-section, while simultaneously a large portion of the superimposed heat and conversion stresses are decreased in the martensitic border zone because the temperature once again increases up to the maximum MS temperature.
As is well known, for realizing comparatively low final rolling temperatures with stand spacings which are not too large, a cooling of the roll body surfaces which is as intensive as possible is required in addition to the use of water cooling sections between the individual stands.
The principal reason for roll cooling is that the roll may not exceed a certain temperature in order to maintain the strength of the roll surface and, thus, the service life of the roll. Another reason is the diameter tolerance of the roll which is only ensured when the roll temperature is constant.
In accordance with the prior art, a conventional roll cooling is carried out by spraying water from spray nozzles onto the roll. This requires large quantities of water in order to keep the low temperature level of a roll at 60 to 80.degree. C. However, the removal of these quantities of water pose a significant problem because the water which flows off may be backed up and may prevent the liquid being supplied from unimpededly reaching the roll surface, so that the cooling effect is significantly reduced.
In order to eliminate this problem, DE 36 16 070 C2 proposes a so-called gap cooling system in which cooling water is fed through a feed opening arranged at the lower end of the cooling body into a cooling gap which partially surrounds the circumference of the roll body and is discharged out of the cooling gap through a discharge opening arranged at the upper end of the cooling body. This arrangement has the disadvantage that sealing of the cooling gap is effected by placing the cooling body of elastic material under pressure against the roll. This may lead to damage of the roll casing, for example, due to abrasion, so that markings on the rolled strip may be produced.